The present invention relates to disc brakes for vehicles, and in particular to an arrangement for connecting and securing a brake disc to an axle hub, including axle hubs utilized on commercial vehicles such as tractor-trailer trucks, box trucks, buses, and the like. The invention also relates to a method for installation of a brake disc on an axle hub.
Disc brakes are increasingly being used on commercial vehicles, replacing conventional drum brakes. Very high braking energy is generated when the disc brake's caliper applies the brake pads to the brake disc to slow such heavy vehicles. In order to deal with such loads, very robust and often complicated designs have been required to connect the brake disc of a disc brake to an axle hub to transfer the braking forces from the brake disc to the hub. The design of the brake disc-to-hub connection is further complicated by the heal generated during braking as the kinetic energy of the vehicle is converted into heat energy by application of the brake pads to the brake disc. The heat the hub receives from the brake disc can be detrimental to the axle hub and its components (such as bearings and seals), as well as causing high component stresses due to differences in thermal expansion between different materials (for example, between an aluminum hub and a steel brake disc). The high heat can also cause brake fade and contribute to premature failure of braking components.
Commercial vehicle brake discs, also referred to as “brake rotors” or “rotors,” often are mounted onto axle hubs using so-called spline arrangements using a fixed or floating connection, such as taught in U.S. Pat. Nos. 6,626,273 and 7,410,036. One example a semi-floating connection is the Splined Disc® brake assembly from Bendix Spicer Foundation Brake LLC. These types of brakes typically are mounted on an axle hub having a plurality of axially-oriented splines arranged around an outer circumference of a disc-mounting region of the hub. The brake disc has corresponding radially-inward facing tabs about the inner circumference of the brake disc. The disc is mounted to the axle hub by axially sliding the brake disc onto the hub's mating splines, followed by insertion and/or attachment of a variety of fasteners, brackets, etc., as necessary per the particular splined disc's design in order to secure the brake disc against axial movement off of the hub. When so mounted, the brake disc's tabs engage the hub's splines in a manner which permits the very large braking forces generated by the disc brake to be transferred to the axle hub and hence to the axle to slow the vehicle. This often requires costly precision machining of the spline/lab engagement surfaces.
Splined discs typically have had substantial metal-to-metal contact between the inner radial tabs of the brake disc and either the faces of the axle hub splines or intermediary inserts that are used to transfer the braking loads from the disc tabs to the hub splines. The intermediate inserts are used in conjunction with hub axial stop to axially restrain the brake disc on the axle hub. This metal-to-metal contact has the disadvantage of facilitating transfer of a large amount of brake heat from the brake disc directly to the axle hub. This is a particular problem where the axle hub is formed from aluminum, a material which is being more frequently used for axle hubs in order to minimize vehicle weight and improve fuel economy, both because the material properties of aluminum (e.g., strength) are known to degrade at higher temperatures, and because the aluminum of the axle hub and the material of the brake disc (typically cast iron) can have significantly different thermal expansion coefficients.
Other brake disc mounting arrangements are known which fix the brake disc to a hub or only allow limited relative movement between the brake disc and the hub. Such arrangements can inhibit the radial expansion of the brake disc, hub and connecting elements, leading to problems such as brake disc deformation (for example, “coning” of the brake disc, in which the friction surfaces of the brake disc bend out of a plane perpendicular to the axle hub's rotation axis). Such deformations can decrease brake disc and brake pad life, and cause brake disc “cracking” due to deformation-induced tensile stress.
Prior art brake disc mounting approaches have also had the problem of requiring complex and costly assemblies of shims and/or springs at the hub/disc interface to flexibly take up component clearances provided between the brake components to accommodate differential thermal expansion and wear- and noise-inducing vibrations. Further, the need to provide disc-to-hub joints that are robust enough to be able to withstand very high temperatures during braking events and metal fatigue over the extended life of a brake disc has required the use of brake discs with undesirably high mass and/or complexity and cost, such as the forming (typically by casting) of a tough material such as ductile iron over the grey iron of the brake disc in the hub region of the disc.
There exists a need for a brake disc mounting arrangement which substantially reduces or eliminates altogether the need for complex shim and or spring assemblies, is simple to assemble, can withstand high heat loads with as low a thermal mass as possible, resists brake disc deformation and uneven brake disc and brake pad wear due to differential heat-generated disc coning, is able to accommodate free radial thermal expansion with little or no binding between the brake disc and hub, and provides a fatigue life which exceeds the design life of the brake disc.
In order to address these and other problems with brake disc mounting in the prior art, the present invention provides a brake disc having a hub region geometry which accommodates differential radial growth of the axle hub and the brake disc, minimizes the number of, or eliminates entirely, the need for individual intermediary disc-to-hub elements, is simple to assemble and disassemble during installation and/or replacement of the brake disc, minimizes the impacts of torsional vibrations without the need for an additional vibration damping mechanism, and is cost effective.
In one embodiment of the invention a brake disc is provided with a plurality of transverse wedge-shaped slots about an inner circumference of the brake disc which are formed with a specific geometry which substantially reduces the stresses in the radially-inward-facing disc teeth between the wedge-shaped slots.
The brake disc slots are radially positioned in locations corresponding to brake disc mounting studs provided on an axle hub. The brake disc and the hub are connected to one another by wedge-shaped elements (aka “keys”) that are positioned in corresponding transverse wedge-shaped slots or holes in a radially inner region of the brake disc, preferably with a retaining device that captures the portions of the brake disc between adjacent keys against axial movement away from the axle hub. The brake disc's wedge-shaped slots may be open on the radially inward side of the slot, or may be closed on the radially inward side, forming generally key-shaped holes at the inner radius of the brake disc.
The keys are provided with an aperture that can pass over a respective brake disc mounting stud, and with side surfaces that conform to the inner surfaces of the wedge-shaped brake disc holes. The keys may be formed from any material that can withstand the forces and temperatures encountered during braking events in this region of the hub and brake disc, and preferably from a material which is corrosion-resistant in the harsh environment of an axle hub.
Preferably, the contact surfaces between the lateral sides of the keys and the lateral sides of the wedge-shaped slots are sized large enough that, given the selected key and brake disc materials, contact surface deformation and wear are minimized to the point that intermediate shims, springs or other contact surface-protecting devices are not needed, i.e., such that the materials and geometry of the keys and the brake disc permit direct key-to-wedge-shaped slot contact without intermediate devices such as spacers and/or spring elements while still providing a long service life without premature wear or damage to the key and slot contact surfaces. The precise design of the geometry of the complementary keys and wedge-shaped slots also permits elimination of the use of intermediate vibration damping devices between the keys and the wedge-shaped slots, as the inherent rigidity of the present invention's “gap-driven design” ensures the resonance frequency of the assembly is relatively high (for example, above 200 Hz) and therefore out of the range of the natural frequencies of the vehicle's wheel-end components (natural frequency being a function of mass and stiffness of the components).
Preferably the sides of the wedge-shaped keys and their respective brake disc holes have their circumferential sides (the sides between their radially inner and radially outer sides that are approximately parallel to the hub rotation axis), generally aligned in the direction of radii extending from the hub rotation axis. Arranging the key and hole sides in this manner facilitates cooperative movement of the keys in their holes during simultaneous thermal expansion of the hub and the brake disc, thereby minimizing the potential for jamming between the keys and the brake disc and resulting thermally-induced stresses in the hub/disc system. Other geometries are possible as the wedge geometry is a function of the thermal mass of the rotor (the heat source) and the vane structure (dissipating heat).
Preferably, the sides of the wedge-shaped slots and the keys are arranged with an angle relative to the radii in the range of 12° to 20°, more preferably 16°. As compared to conventional brake discs with parallel slot sides (aka “straight teeth”), a brake disc having slot side angles in the preferable range surprisingly has a stress distribution around the circumference of the disc's inner hub attachment region during it braking event which is substantially more equally distributed between the inward-projecting disc teeth than in a brake disc with straight teeth.
It is known in the art that when brake pads are applied to a brake disc during a braking event, the pad's clamping forces are applied over a limited arc of the friction surfaces of the disc. As a result, the amount of the braking load sustained by the individual teeth varies with the number and circumferential position of the teeth about the hub. For example, in a brake disc with ten straight teeth, the tooth carrying the highest load may be carrying 10 times as much load as a diagonally-opposite tooth. A brake disc with ten wedge-shaped slots in the preferred slot-side angle range instead may see maximum-to-minimum load difference ratios of less than 3:1. The much more even sharing of the braking force loading among the brake disc mounting interface has several benefits, including lower maximum stress levels, reduced contact surface wear and longer component life, and the ability to design smaller brake disc interfaces which have less contact area for heat transfer from the brake disc to the hub.
Preferably, the keys are sized in the axial direction such that they are firmly biased against the hub at all times. The holes in keys through which fasteners pass preferably are sized near the size of the outer diameter of the fastener in order to maximize the load-bearing surface contact between the keys and the fasteners.
The present brake disc mounting arrangement is particularly simple and easy to install and/or replace. An embodiment of a method of installation includes locating a brake disc on an axle hub with the brake disc's wedge-shaped holes aligned with the hub's mounting studs or fastener-receiving holes, inserting corresponding wedge-shaped keys into the brake disc's wedge-shaped holes, placing u bolting ring over the keys, and installing fasteners that bias the keys against the hub. The keys allow the rotor to be piloted on the hub. Other variations are possible, for example, the keys may be located in the brake disc holes before the brake disc is located on the axle hub, or the fasteners may be fed through the keys before the keys are located in their respective brake disc holes.
The present invention further has the commercially significant advantage of providing the ability to readily adapt different brake disc designs from various brake component manufacturers to mount the brake discs on any standard flat-faced axle hub from various axle hub manufacturers.
There are multiple axle hub designs in the market, each with an associated component for supporting a brake caliper known as a “torque plate.” The torque plate typically defines, in a restrictive manner, the location of the brake caliper and its carrier relative to the hub. The brake caliper and carrier design in turn defines the axial location of the brake disc rotor, which must be located between the brake pads on which the brake caliper's brake applications devices act to apply the brake. The axial location of the brake disc can be a critical parameter. The tight clearances in a commercial vehicle wheel hub region raises concerns for maintaining adequate clearance to wheel valve stems to avoid impacts which could shear off a valve stem and cause sudden tire deflation. The tight spaces also raise concerns with the brake actuator not being misaligned to the point of hitting the frame and accidently releasing a parking brake.
Due to the variety in proprietary brake component designs, there is no “universal” brake disc in the commercial vehicle market which may be mounted directly to all, or even most, axle hubs (due to, for example, different boll patterns) and which will be assured of being in the correct axial location for caliper fitment in different brake designs.
The present invention provides the opportunity to provide a brake disc mounting arrangement compatible with a universal or near-universal brake disc by providing appropriately-dimensioned key rings that correctly male a brake disc with the present invention's key-receiving slots with a particular combination of axle hub and brake caliper designs. For example, in many applications one or more manufacturers may supply components for a wheel end that includes a particular model of an axle hub, a particular model of a torque plate, a particular model of a brake caliper, and a particular model of a brake disc with a mounting fastener pattern and axial offset to suit that unique combination of components. When it is time to replace the brake disc, rather than being required to use a proprietary brake disc, the a standardized (and thus lower cost) brake disc with an appropriate key-receiving slot arrangements may be adapted to the particular brake application. Such a standardized brake disc may be mounted to the particular axle hub using an intermediate key ring adapter that is dimensioned with mounting pattern that is compatible with the particular hub's mounting stud pattern (i.e., a particular pattern of stud holes at a particular mounting hole ring radius). The associated key ring would be provided with an appropriate thickness to ensure the standardized brake disc is properly axially aligned with the particular model of brake caliper (which in turn is axially located by the particular model of torque plate). The axial offset of the brake disc from the face of the particular model of axle hub may be readily set by making the key ring's webs between adjacent keys the appropriate thickness that results in the brake disc being correctly positioned between the caliper's brake pads when the brake disc abutting the key ring webs. In other embodiments, the brake disc and/or the key ring webs may be provided with more than one axial height, such that by rotation of the brake disc relative to the ring during installation, different axial positions of the brake disc relative to the torque plate may be obtained.
In the prior an there are known to be hundreds of combinations of torque plate, hub, brake caliper and brake rotors and associated offsets. The use of a limited number of standardized brake discs with appropriate key ring adapters would enable significant cost savings from simplified and more efficient brake disc manufacture (lower tooling costs and cost efficiencies from greater production volume as compared to more limited production of individual proprietary brake disc designs), simplified product logistics (fewer part numbers to administer and maintain in inventory, and greater availability to immediately fulfill a parts order); and simplified and less costly service needs (less technician time to determine what parts are required for a particular brake service and to complete the service).
Preferably, the key ring is formed from a powdered metal, which offers several advantages over aluminum and other materials such as steel alloys.
This approach reflects a substantial departure from the prior art.
In the prior art the conventional wisdom has been that costly materials with higher elongation and higher yield strength properties had to be used in an application such as the present invention, in order to increase fatigue life and otherwise provide sufficient resilience to survive the high temperature, high vibration, high applied force environment of a commercial vehicle disc brake.
Counter to this conventional belief, the inventors have deliberately selected a more brittle material with a low range of elongation, applying the material in a highly targeted manner, such as varying the powdered metal's density in different regions of a key ring to provide higher Strength only in regions where needed. The use of a more brittle material is further aided in applications with the above-described key-and-slot arrangements, as the lower peak stresses experienced by the brake disc during a braking event provides additional design margin, i.e., lowers the stress levels the powdered metal must be able to withstand.
Powdered metal component properties are highly dependent on the process and equipment used to form the component, where the properties of the material of the component are functions of surface area, press force, material alloy composition, and the combination of the shaping of the component mold and the distribution of the powdered metal within the mold prior to compacting. For example, when a powdered metal alloy composition of FLC-4805-100HT per MPIF Standard 35 is subjected to compression in a 750 ton press, a targeted density on the order of 7 grams/cm3 may be obtained in a key ring with a surface area of 115 cm2. In a specific example of a particular key ring (i.e., without limiting the present invention to the specific numerical values that follow), the powdered metal may have a targeted range of material densities on the order 6.9 gr/cm3-7.2 gr/cm3, with the density made higher in critical areas, such as at a radius between a key and inter-key web (i.e., in a stress concentration region). At a post-formation density of 6.8-7.0 gr/cm3, the local yield strength will be on the order of 725-760 MPa in the high-stress root region, which is substantially higher that the maximum loading expected in this particular key ring (560 MPa).
The use of variable-density powdered metal as a brake disc-to-axle hub adapter material provides many advantages, and frees designers from the prior art's material constraints. With targeted powdered metal design, designers may now develop adapter designs in which the engineering requirements (e.g., strength, fatigue life, fracture toughness) can be met while meeting other priority demands such as lower cost and weight.
A powdered metal key ring in accordance with the present invention can be expected to be lighter than a key ring formed from a steel alloy that can meet the same strength requirements. This represents the potential for substantial savings in weight at each axle end (contributing to improved fuel economy and consequently lower emissions), as well as savings in cost from avoiding use of high-cost alloy steel materials and difficult machining operations.
The powdered metal key ring of the present invention also avoids the problems of some conventional lighter-weight materials. For example, it is well known that at higher temperatures (temperatures obtainable in a heavy braking environment) aluminum loses a significant portion of its strength. As a result, components formed from aluminum must be designed accordingly, which typically resulting in much larger components to lower the local stresses to a survivable range (and thereby negating much of aluminum's weight advantage). In contrast, powdered metal's material properties are significantly less temperature dependent over large temperature ranges; indeed, powdered metal sintering temperatures are far above any temperature likely to encountered in a braking environment. Powdered metal components may also be designed to be substantially smaller than corresponding aluminum components, as powdered metal is typically on the order of five times stronger than aluminum.
From a thermal isolation standpoint, a powdered metal key ring may provide further “downstream” benefits. For example, because powdered metal is a good thermal isolator, the amount of heat transferred from the brake disc to the axle hub through the key ring may be lower than the amount of heal that would be otherwise transferred in a conventional brake disc mounting arrangement. This in turn may translate into the ability to use aluminum as the axle hub material in place of heavy iron or costly steel, because the aluminum hub would be less likely to see temperatures high enough to unacceptably reduce the strength of the aluminum. A further benefit may be significantly reduced temperatures at the bearings on which the hub rotates.
The benefits of the thermal isolation capabilities of a powdered metal brake disc key ring adapter are exemplified by a comparison with prior art brake disc designs. In the prior art, in order to prevent temperatures in the material of a flat-faced axle hub from exceeding design limits, a common solution was the so-called “U-shaped” brake disc, i.e., a brake disc having friction discs (the region of the highest temperature during a braking event) that are held axially well away from the face of the axle hub by a “hat” or bucket-shaped flange section (in cross-section, U-shaped sections). To the knowledge of the inventors, the use of a key ring adapter formed from variable-density powdered metal, particularly use such an adapter with the stress-equalizing geometries discussed above, has resulted in the first practical, cost-efficient design that can provide thermal isolation comparable to a U-shaped brake disc. In one example, the present key ring adapter approach resulted in temperatures at the bearings of an axle hub on the order of 50° C., well below a design target of 60° C. and far below the temperatures on the order of 80-90° C. with a prior flat rotor attachment approach.
Powdered metal also has advantages in lower cost and simpler component manufacturing operations. Powdered metal components are formed in “net shape” or “near net-shape” processes, primarily by high-pressure, and optionally high temperature, pressing in molds. When the components are removed from the molds they are in a near-finished state, thus avoiding costly, intricate machining such as that required of raw, unfinished forged component cores.
The powdered metal key ring in accordance with the present invention also provides related advantages during initial installation and subsequent replacement of brake discs. A large fraction of the prior art brake disc mounting arrangements require the use of additional small parts, from simple to complex combinations, to secure and/or prevent transfer of vibration energy between the brake disc and the axle hub and vice-versa. These spring and/or shim components add cost to the brake design, and require additional technician effort and time (with its related labor costs) to complete disassembly and reassembly of these components during a brake disc replacement job. All of this costly hardware and labor is eliminated by present invention, where the key ring adapter may be placed directly on the hub face, the brake disc placed on the key ring, with a simple cover ring capturing the brake disc on the key ring.
The scope of the present invention further includes alternative embodiments which similarly permit a “universal” or common rotor to be fitted to existing hubs while flexibly being able to accommodate different brake disc or rotor axial positions. For example, an inner surface of the key ring and/or a axial collar of the key ring may be provided with internal threads configured to engage corresponding external threads on an axial surface of a hub. Coupled with a relatively thin locknut also threaded onto the hub's external threads, the key ring could be rotated to a desired axial position and then locked into place by tightening the locknut against an axial face of the key ring. In addition to providing essentially unlimited positioning variability in the axial range of the overlapping threads, this arrangement may provide a particularly axially-narrow brake disc mounting solution.
Alternatively, for existing designs in which the hub is not equipped with external threads, an externally-threaded adapter base may be secured to the face of the hub using the hubs existing fasteners (e.g., studs and nuts or bolts that screw into bores of the hub). A locknut and an internally-threaded intermediate key ring as the previous embodiment may then be installed in the same manner on the adapter bases external threads.
A further embodiment may have the axial height adjustment capability of the present invention embodied in a manner that does not require either the hub or an adapter base and the key ring to have corresponding internal and external threads. For example, an adapter base without threads may receive leadscrews that axially project toward the key ring, which in turn receive threaded collars. The collars may be configured to axially receive the key ring, with the axial position of the key ring being adjustable by rotating the threaded collars along the leadscrews until the desired axial position is reached. The threaded collars may then be locked into place, for example by using jam nuts threaded onto the remaining projecting threads of the leadscrews.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
Common reference label numbers are used with common features in the figures.